Desalting, desalination, or desalinization refers to water treatment processes that remove salts from water. These water treatment processes are typically used to produce potable or drinking water from seawater or brackish water. Commercially available desalting processes may be categorized into major processes and minor processes. Major processes include temperature-driven or thermal processes and pressure-driven or membrane processes. Thermal processes include multi-stage flash distillation, multiple-effect distillation, and vapor compression. Membrane processes include electrodialysis and reverse osmosis. Minor processes include freezing, membrane distillation, and solar humidification.
Scale formation and deposition is a significant problem in equipment used in desalting processes. Scale forms when the concentration of a dissolved mineral exceeds its solubility limit and the mineral precipitates. The temperature, pH, water composition, and residence time, for example, of the thermal desalination process determine the types of scale likely to form and thus the scale control requirement. At lower temperatures (e.g., from about 80° C. to about 85° C.), calcium carbonate (CaCO3) scale typically forms. Temperatures from about 90° C. to about 95° C. generally cause Mg(OH)2 scale. Typically, calcium sulfate (CaSO4) is a major contributor to scale formation when seawater or salt water approaches about 100° C. or higher. CaSO4 material forms a hard scale that coats any tubes or surfaces present. Scale creates thermal and mechanical problems and is sometimes difficult to remove, especially in the case of CaSO4 scale. Membrane processes typically have scale formation problems in the reject stream and membrane fouling due to scale build-up.
A known method of preventing scale formation includes controlling the seawater concentration levels (recovery) and temperature in thermal desalination processes. Multi-stage flash distillation plants usually operate at temperatures from 90° C. to 110° C. and operating the process at higher temperatures increases the efficiency, with a concomitant increase in the potential for scale formation and accelerated corrosion of metal surfaces. Chemical scale inhibitors are also used in both thermal and membrane processes. Commercially available examples include Belgard® and Flocon®, from BWA Water Additives, and Sokalan® PM10 and PM15, from BASF—The Chemical Company.
Reverse osmosis end users frequently employ chemical pretreatments, such as the addition of polymeric scale inhibitors/dispersants to inhibit undesirable mineral scaling. In some cases, inorganic inhibitors, such as sodium hexametaphosphate (SHMP), are used. For example, U.S. Pat. No. 4,563,284 discloses a method for inhibiting formation and deposition of scale-forming salts by adding a phosphonocarboxylic acid and a telemetric phosphinecarboxylic acid that contains features of both phosphonates and polyacrylates. U.S. Pat. No. 4,762,621 discloses a scale inhibitor comprising a copolymer of an acrylic acid and a lower alkyl ester of itaconic acid. U.S. Pat. No. 4,784,774 discloses a scale inhibitor containing a homopolymer of maleic acid or a copolymer of a monounsaturated monocarboxylic or dicarboxylic acid or salt thereof containing 3 to 5 carbon atoms and a phosphonoalkane carboxylic acid.
Thus, a major operating problem in seawater distillation plants and reverse osmosis systems is the formation of mineral scales, which reduce heat transfer, plug tubes, foul membranes, and ultimately lower the rated capacity of a plant and increase costs. As industry recognizes the need for new scale inhibitors, there exists a need to develop improved methods of using scale inhibitors.